Processes and structures for IC fabrication

ABSTRACT

The present invention discloses methods and apparatuses for the separations of IC fabrication and assembling of separated IC components to form complete IC structures. In an embodiment, the present fabrication separation of an IC structure into multiple discrete components can take advantages of dedicated IC fabrication facilities and achieve more cost effective products. In another embodiment, the present chip assembling provides high density interconnect wires between bond pads, enabling cost-effective assembling of small chip components. In an aspect, the present process forms interconnect wires on a thermal decomposable adhesive, and after positioning the wires at proper bond pad locations, releases the interconnect wires onto the bond pads.

The present invention relates to co-pending applications “Processes andstructures for IC fabrication” of the same inventor.

BACKGROUND

RFID tags are a class of integrated circuits which intercept modulatedRF signals from a transmitter (which is normally called a “reader”),demodulate the signal and apply the encoded information to a digitalportion of the circuit, and radiate a modulated signal to the reader.The signal typically contains only a small amount of information such asthe identification number of the device, although more information maybe stored in the tag's memory and may be used either to determinewhether the tag responds, or to influence the string of bits which areapplied to the modulator. The modulator alters the Q of the antenna andthus its normally re-radiated signal, so no active power is applied tothe transmission.

Such tags are intended to be applied to a very large number of objects,for example to shipping boxes or even to the individual items in a store(this application is called “item level tagging”). In order not tounacceptably increase the cost of the objects, the tags have to be verycheap: for item level tagging, for example, the cost to the user isdesired to be under 5¢.

In order to minimize the total tag cost, each component should becarefully controlled, including the integrated circuit (chip), theantenna, the packaging (substrate and encapsulation), and the process. Atypical cost of IC manufacturing for many years has been approximately$4/cm², or 4¢/mm². Clearly if the IC is 1 mm² in area, it will not bepossible to meet this target. Yet, the RFID chips available, fromcompanies such as Texas Instruments, NXP Semiconductors, Impinj, etc.are typically between about 0.5 and 1 mm², with most on the high side(or even higher), and actual chip costs (even in high volume) arecurrently somewhat higher than 4¢. Clearly there is a need for smaller,and hence less expensive, chips.

SUMMARY

The present invention relates to processes and structures of integratedcircuits (IC), particularly to radio frequency identification (RFID)tags, and more specifically to passive (non-powered) RFID tags. In anembodiment, the present invention discloses methods and apparatuses forthe separations of IC fabrication and assembling of separated ICcomponents to form complete IC structures. In the fabrication of ICdevices, various fabrication processes and materials are not compatible,and many fabrication facilities are not equipped to handle all neededprocesses and materials. The present fabrication separation of an ICstructure into multiple discrete components can take advantages ofdedicated IC fabrication facilities and achieve more cost effectiveproducts.

In an embodiment, the present fabrication separation separates thefabrication of analog portions from the fabrication of digital portionsof an IC device. Digital fabrication processes have been advancedsignificantly and digital fabrication facilities can produce submicrondevice IC with very high device density. In many cases, digitalfabrication cost is thus directly related to the chip size. In contrast,analog fabrication processes provide lower device density with largerchip size. However, the cost per chip area for analog fabrication can besignificantly lower than a same chip area for digital fabrication. Thusthe present fabrication separation can optimize the cost of an IC thatincludes both analog and digital portions, separating the digitalportion and the analog portion to achieve a small chip area for thedigital portion and a higher chip area for analog portion. In addition,the present separation process can take advantages of dedicated digitalfacilities, which might not be equipped for routine processing of analogdevices.

In an embodiment, the present separation brings to digital fabricationall processes and device structures that are common to MOS or CMOSprocesses, such as planar transistor devices that have substrate singlecrystal silicon channel. Other processes related to common MOS or CMOSprocesses include silicon dioxide gate dielectric, silicon oxide orsilicon nitride spacer or passivation layer, impurity ion implantationfor doping source/drain regions and gate electrode, polysilicon gateelectrode, silicidation and aluminum metallization. The presentseparation can bring to analog fabrication all analog processes andstructures, such as power devices, RF devices, thin film devices, orpassive components. For example, the analog portion can comprise passivecomponents constructed with dielectric other than silicon dioxide and/orsilicon nitride, or with materials not common to MOS or CMOS processes.The analog fabrication portion can comprise capacitor structures thatemploy materials different than silicon dioxide, or materials with highdielectric constant (dielectric constant higher than that of silicondioxide). Analog fabrication portion can comprise resistor structuresthat employ materials different than doped polysilicon. Analogfabrication can also comprise active components constructed with non-MOSor non-CMOS process, for example, devices with non-single crystalsilicon channel, such as polysilicon channel or III-V or II-VI channel.

In an embodiment, the present separation removes the portions of theintegrated circuit that are not common to a digital processing facility.Thus the portions of the IC that can be fabricated in a digitalprocessing facility can be arranged to achieve optimum costeffectiveness for the whole IC structure. For example, devices thatemploy new materials, such as capacitors with high dielectric constant(BST, for example) can be removed from the digital fabrication facility,and processed in an analog facility.

In an embodiment, the present separation removes the portions of theintegrated circuit that have critical dimensions greater, or 2× to 5×greater, than that of a digital processing facility. In an aspect, thepresent separation removes the portions of the integrated circuit thatcan affect the chip area of the digital portion to achieve the smallestdigital chip size, and thus smallest cost for digital processing. Forexample, for a 0.25 μm digital fabrication facility, active devices arefabricated with 0.25 μm processes, and smallest chip area can beachieved for devices using 0.25 μm critical dimension. Thus otherdevices with larger dimension, for example, capacitors, resistors, powertransistors or polysilicon/deposited or printed single crystalsilicon/amorphous transistors, can be removed from the digitalfabrication facility, and processed in an analog facility.

In an embodiment, the present fabrication separation separates the ICinto separate functional components, such as memory, multiple digitallogic portions, power devices, RF devices and passive devices. Thecomponents are preferably small, for example, 1 mm or less, and thin,for example, 100 μm or less. The components are also preferablyassembled close to each other, for example, less than 100 μm, orpreferably less than 10 μm. In an aspect, the components do not requireany packaging before assembling, for example, assembling in wafer form.In an aspect, the components are assembled with a thin film transferprocess, which can handle small and thin components. In an aspect, thepresent invention discloses an assembling process, stacking thecomponents in stacked configuration, partially stacked configuration,coplanar configuration, partially coplanar configuration, or anycombination thereof. In an aspect, the small sizes of the functionalcomponents provide better flatness and uniformity of the contact pads,thus the connection technology for these small functional components isgreatly simplified. For example, flip chip connection can be used withsmaller spacing.

In an embodiment, the present fabrication separation separates the RFIDtags, forming separate digital portions from analog portions. In anaspect, the analog portions of the RFID tag comprise active devicesconstructed in non-MOS or non-CMOS process. The analog portions cancomprise active devices having non-substrate single crystal siliconchannel, such as polysilicon channel, deposited or printed singlecrystal silicon channel, or GaAs channel. The analog portions cancomprise thin film transistors, such as transistor having depositedchannel regions. In an aspect, the analog portions of the RFID tagcomprise passive components constructed in non-MOS or non-CMOS process.The analog portions can comprise passive components constructed withdielectric other than silicon dioxide or silicon nitride, for example,capacitor devices with dielectric material having high dielectricconstant, or resistor devices with conductive material different thandoped polysilicon. In an aspect, the RFID tag is separated into memorycomponent, digital logic component and RF components, which are thenconnected to form the RFID tag.

The present invention further discloses processes and structures forinterconnection assembling of the separate components to form a completeintegrated circuit. In an embodiment, the present invention disclosesgradual slope edge of the top component, for example, formed by a laserablation process. When a top component is assembled on top of a bottomcomponent, the sharp edge of the top component might cause difficultywith the interconnect wires connecting the top component to the bottomcomponent. With a gradual slope, interconnect wires can be easilybonded, and the bonding reliability can be improved. The gradual slopecan be formed along the edge of the component, or can be form along theinterconnect wire, in the direction from a bond pad of the top componentto the bond pad of the bottom component.

In an embodiment, the present invention discloses simultaneous bondingof multiple bonding pads to the interconnect wires. For example, acomponent typically has multiple bonding pads arranged next to eachother at the edge. These bond pads can be prepared to simultaneouslybond to multiple interconnect wires. In an aspect, conducting adhesivecan be applied continuously to these bond pads, with the adjacent bondpads electrically connected to each other by the adhesive. Multipleinterconnect wires then can be applied to these bond pads at the sametime, forming multiple bonds from the multiple interconnect wires to thecorresponding multiple bond pads. A bond pad separation process such asa laser cut can be applied to separate the adjacent bond pads. Inanother aspect, multiple interconnect wires can be positioned on theseadjacent bond pads, and a laser welding can travel along these bondpads, welding all these interconnect wires to the corresponding bondpads, together with some connection between adjacent bond pads. Aseparation process, such as a laser cut, can be applied to separate theconnection between the adjacent bond pads.

In an embodiment, the present invention discloses laser cutting and/orlaser welding to bond the interconnection wires to the bond pads, whichcan provide high density wire bonding due to the small size of the laserbeam. In an embodiment, the present invention discloses interconnectionprinting using thermally decomposable adhesive. The presentinterconnection printing process comprises forming interconnect wireswith correct lengths on a thermally decomposable adhesive on a printheadsubstrate, placing the interconnect wires on proper bond pads byreleasing the thermally decomposable adhesive, and then laser weldingthe interconnect wires to the bond pads. An optional force element canbe used to push one or more interconnect wires down into contact withthe bond pads to facilitate the laser welding process. In certain cases,the laser welding process forms a liquid at the bond pad and/or softensthe metal wire, which then facilitates the contact by surface tensionand gravity. For contacts between stacked chips, the top chip can form aslope to avoid a sharp chip edge and facilitate interconnect wirealignment.

In an aspect, narrow wires are formed on the thermally decomposableadhesive, and laser cuts can be performed to prepare the wires to thecorrect length. In another aspect, a metal film is deposited on thethermally decomposable adhesive, and then be patterned into metal wires.The patterned metal wires can be patterned to the correct length, or anadditional laser cut can be performed to cut the patterned wires. Inanother aspect, a conventional wire bonder system can provideinterconnect wires to the thermally decomposable adhesive. The wirebonder can feed, cut and place wires with correct length on thethermally decomposable adhesive, utilizing the capabilities of the wirebonder. Alternatively, laser cutting can be used to cut the wires.

In an aspect, multiple wires can be released simultaneously to thecorresponding bond pads. For example, the multiple bond pads areprepared to accept bonding to multiple wires, such as by applying acontinuous strip of conductive adhesive along the adjacent bond pads.Alternatively, after the multiple wires are positioned on thecorresponding bond pads, laser welding can be applied to continuouslyweld the multiple wires to the corresponding multiple bond pads. Thelaser can jump from one bond pad to the next bond pad. Alternatively,the laser can weld continuously from one bond pad to the next bond pad.In this case, there might be some electrical connection between adjacentbond pads. A laser cut might then be applied to separate the potentialconnection of adjacent bond pads. After the connection of one end of themultiple wires, the other end can be bonded to the corresponding bondpads.

In an embodiment, a conventional wire bonder can be utilized togetherwith laser welding and/or laser cutting. In an aspect, the laser weldingreplaces the typical ultrasonic welding of the wire bonder, providingthe capability to bond much smaller wires. The wire bonder can providethe capillary, feeding mechanics and positioning technology, all ofwhich can be performed on much smaller wires than those currentlyavailable. The wire bonder can be used to extrude a desired length ofthe wire over a desired location, where a laser cut allows the wire tofall into position. The laser, or another laser, is used to weld theends of the wires.

In an embodiment, the present invention discloses simultaneous formationof multiple interconnect wires in the form of a ribbon. A ribboninterconnect is bonded to multiple bond pads at both ends of the ribbon.A separation process, such as a laser cut, along the length of theribbon and between the adjacent bond pads is performed to separate theribbon into multiple interconnect wires, connecting the correspondingbond pads. A beveled slope can be applied to the top component tofacilitate the formation of the ribbon interconnect. The present ribboninterconnect can be applied to stacked components, partial stackedcomponents, or side-by-side components.

In an aspect, a conductive adhesive is applied to the bond pads, eitherto the multiple bond pads at one end or both ends of the ribboninterconnect, before placing the ribbon interconnect on the bond pads.The conductive adhesive can be applied (e.g., printing) continuously,along a direction substantially perpendicular to the length of theribbon, bridging the bond pads from one end (or each end) of the ribboninterconnect. Both the ribbon and the conductive adhesive line connectthe bond pads together. A laser then may be used to form cuts along thelength of the ribbon, separating the ribbon into substantially parallelindividual wires. The resulting wires may be encapsulated for protectionand stability, for example, by applying a droplet of curable adhesive.

In an aspect, a welding process is performed to bond the ribbon to thebond pads, after placing the ribbon interconnect on the bond pads. Allthe bonds may be formed in a single ultrasonic welding operation, or thebonds may be formed by a laser welding. The laser welding process canaddress each pad in sequence, continuously or jumping between pads. Themovement of the laser welding process along the bond pads can beperformed by a mechanical motion moving the laser beam, or from a movingmirror mechanism. A separation process, such as a laser cut, is thenperformed to separate the ribbon into multiple individual wires. Anencapsulation process can also be applied.

In an embodiment, the present invention discloses a surface preparationprocess to facilitate the bonding process between stacked components(such as dies), wherein the surface of at least one of the components istreated in such as a way so that a liquid metal will not wet or stick toit. This surface treatment process is applied to the whole surface,excluding the bond pad areas. This surface treatment can preventshorting between bond pads, helping to confine the bonding to the bondpad areas. When the two components are pressed with the bond pads facingeach other, the liquid metal can make a thin and uniform electricalcontact between the two bond pads. In an aspect, the liquid metal can bemixed with suitable particles which alloy to form a higher melting pointmetal, which then forms a bond as the alloy reaction proceeds.

In an embodiment, the present invention discloses encapsulated liquidmetal for bonding surfaces, wherein capsules or vesicles of a liquidmetal are surrounded by a polymer shell. The polymer shells can beformed from an emulsion process, or by self-assembly of polymer chainsas surfactant. In an aspect, the polymer has a specific affinity for thecontact pads, which facilitates the coating of the polymer shells ontothe surface of the components. After forming a layer (such as amonolayer) of these capsules on the contact pads of a component, anothercomponent is placed against these capsules, matching the contact pads ofthe top component to the contact pads of the bottom component. The twocomponents are then squeezed together, breaking the capsules andallowing the metal to react, forming the electrical connection bondbetween the bond pads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary process and structure for separating acircuit diagram into separate components before assembling together toform a complete IC chip.

FIG. 2 illustrates an exemplary process and structure for a RDIF tag.

FIGS. 3A-3C illustrate various exemplary process flows for the presentseparation fabrication process.

FIG. 4A illustrates an exemplary separation process for RFID tag basedon capacitor with high dielectric constant.

FIG. 4B illustrates an exemplary separation process for RFID tag basedon single crystal silicon or polysilicon fabrication process.

FIG. 5A illustrates an exemplary process of separating an IC design intosmall components, and the process of assembling small components to forma complete circuit.

FIG. 5B illustrates an exemplary process of separating a RFID tagdesign.

FIGS. 6A and 6B illustrate cross section and top views of an embodimentof conduction lines connecting a top chip to a bottom chip.

FIGS. 7A and 7B illustrate exemplary configurations of gradual slopes ona top chip.

FIGS. 8A-8B illustrates various exemplary structures and processes offorming the gradual slope on a top chip.

FIGS. 9A-9B illustrates various exemplary flowcharts for a formation ofthe slope to facilitate the connection between the chips.

FIGS. 10A-10D illustrate an exemplary process for simultaneously forminga plurality of interconnect wires.

FIGS. 11A-11D illustrate another exemplary process for simultaneouslyforming a plurality of interconnect wires.

FIGS. 12A-12B illustrate various exemplary flowcharts for the bonding ofmultiple interconnect wires to the corresponding bond pads.

FIGS. 13A-13E illustrate an exemplary process flow for the combinationof wirebonder and laser cutting/welding.

FIG. 14 illustrates an exemplary process flow for the combination ofwirebonder and laser cutting/welding.

FIGS. 15A-15E illustrate an exemplary printing process for interconnectwires.

FIGS. 16A-16C illustrate an exemplary process of preparing the wireswith correct lengths on the printhead coated with a thermal decomposableadhesive.

FIGS. 17A-17D illustrate another exemplary process of preparinginterconnect wires with correct lengths on the printhead coated with athermal decomposable adhesive.

FIGS. 18A-18E illustrate another exemplary process of preparinginterconnect wires with correct lengths on the printhead coated with athermal decomposable adhesive.

FIG. 19 illustrates an exemplary flowchart for the present printingprocess of interconnect wires.

FIGS. 20A-20C illustrate various exemplary processes of forming wireswith correct lengths on the thermal decomposable layer on the printhead.

FIGS. 21A-21D illustrate an exemplary process sequence for the presentribbon interconnect with laser welding.

FIGS. 22A-22D illustrate another exemplary sequence, where the laserwelding runs continuously to bond and connect the adjacent bond pads.

FIGS. 23A-23D illustrate an exemplary process sequence for the presentribbon interconnect with conductive adhesive.

FIG. 24 illustrates an exemplary flowchart for the present ribboninterconnect.

FIG. 25A-25B illustrate various exemplary flowcharts for the presentribbon interconnect.

FIGS. 26A-26C illustrate an exemplary process of the present surfacetreatment.

FIG. 27 illustrates an exemplary flowchart of the present surfacetreatment.

FIGS. 28A-28C illustrate an exemplary process of the present capsulecontact bonding.

FIG. 29 illustrates an exemplary flowchart of the present capsulecontact bonding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses processes and structures of integratedcircuits (IC) fabrication. The following description relates to radiofrequency identification (RFID) tags, but the present invention is notlimited to RFID.

In an embodiment, the present invention separates the analog portionfrom the digital portion of an IC circuit for fabrication. FIG. 1illustrates an exemplary process and structure for separating a circuitdiagram 10 into separate digital component 11B and analog component 12Bbefore assembling together to form a complete IC chip. A circuit design10 can have a digital portion 11A and an analog portion 12A, connectedthrough interconnection 13A. A separation process separates the portionsof the circuit design, according to predetermined criteria, for example,to achieve a low cost of final product. The digital 11A and analog 12Aare fabricated separately in different fabrication facilities 15A and15B, to produce a digital component 11B and an analog component 12B. Thedigital and analog components then undergo an assembly process,including connecting the two portions through interconnect wirings 13Bto produce a final product.

FIG. 2 illustrates an exemplary process and structure for a RDIF tag 20,starting by separating the RFID design 20 into a digital portion 21A, ananalog and/or RF portion 22A and a memory portion 23A with interconnect23A. The three portions are fabricated separately 15 to produce separatecomponents, a memory component 23B, a digital component 21B and ananalog/RF component 22B. The three components are then assembled 16,including providing interconnect 23B, to produce a final RFID tag.

Digital ICs can be made smaller by shrinking the features, and hencefitting the same number of transistors into a smaller area. This type ofshrinking is common in the industry, and is generally referred to as“Moore's Law”. The critical dimension of the digital transistors, i.e.,the dimension of the smallest geometrical features (for example, thewidth of transistor channel, interconnect line, contacts, trenches,etc.) which can be formed during the fabrication of semiconductordevices using a given technology, is smaller and smaller, and the sizeof simple circuitry, e.g., ICs with thousands of transistors, is limitedat the lower end only by the IC handling technology, e.g., pick andplace machine.

However, analog integrated circuits are not subject to the same simplescaling process, because they depend heavily on so-called passivecomponents, which are primarily capacitors, resistors, and inductors.Each of those components has values which depend on fundamentalmaterials physics. For example, the capacitance of a parallel platecapacitor is given by the area of the plates, divided by theirseparation, and multiplied by the dielectric constant. Thus, for a givenset of materials (dielectric), larger capacitances can only be obtainedby making larger area capacitors (thickness reductions are limited bydielectric breakdown). Related constraints apply to the othercomponents. In addition, the semiconductor components used for analogfunctions must carry certain amounts of current (or provide certainvoltages) which are specified by the circuit power and voltage demands,and this places limits on their physical size which results in differentsizes from digital transistors, which only need to provide enoughcurrent to drive the next few transistors, and voltages to switchfollowing gates.

In an embodiment, the present invention discloses a design separationwherein the devices having a critical dimension compatible with thecritical dimension of a digital fabrication facility are fabricated inthe digital fabrication facility. Devices with critical dimension muchgreater than the digital critical dimension are separated and fabricatedin a separate fabrication facility. In an aspect, the present separationprocess separates the digital portion with similar critical dimensionfrom the analog portion with much greater critical dimension to befabricated in different production lines. A criterion for separating theportions of an IC design would be cost saving. For example, byseparating the IC portions based on critical dimension, minimum cost canbe achieved from having components fabricated as densely as possible ona given substrate.

In an aspect, the present invention employs an innovative process ofplacing and connecting two or more different chips in an IC (such as aRFID tag) to achieve lower cost assembly after making them separately.In addition, the present invention provides an incentive to makecomponents as small as possible, because innovative assembly equipment,such as transfer printing with decomposable adhesive, allows picking upand moving smaller components with adequate throughput.

In an aspect, the present invention overcomes some of the shortcomingsof the prior art by making it possible to assemble components of almostany size, down to the micrometer scale. The digital and analog sectionsof an RFID tag can be made separately, on optimized process lines, andcost-effectively combined to make a single, very compact and thin, chipfor assembly into a tag. U.S. Pat. Nos. 6,946,178 and 7,141,348 bySheats, et al., hereby incorporated by reference, describe a method andmaterials for transfer printing by use of a photosensitive thermallydecomposable polymer, which allows the placement of very thin andlaterally small components onto a wide variety of substrates with highprecision. Thus, for example, an integrated circuit chip, which mighthave a thickness of the order of twenty or thirty micrometers (or less),for example, and a width of a few tens to a few hundreds of micrometers,may be placed on top of another integrated circuit chip with sufficientaccuracy that contact pads on one chip will line up effectively withcontact pads on the other. Various methods known in the art, includingconventional flip chip packaging, may be used to form electrical contactbetween the two, as well as methods disclosed herein. Alternatively, theupper chip may be placed face up and interconnection made through viasor over the edge.

The advantage of the invention is that the digital portion of thecircuit may be shrunk according to the scaling process common in theindustry, and thereby reducing the cost according to the area. At thesame time, the analog portion can be fabricated with materials that areoptimized for passive components, which reduces the area occupied (andhence the cost) as well as reducing the cost by using optimizedequipment, simpler processes, and less expensive materials.

FIG. 3A illustrates an exemplary process flow for the presentfabrication separation process. Operation 31A separates an IC designinto different portions, for example, a digital portion and an analogportion, with the analog portion comprising components having criticaldimension greater than the critical dimension of a digital fabricationfacility. The analog critical dimension can be greater, and preferably2-5× greater than the digital critical dimension. The digital criticaldimension is specific to a digital fabrication facility, and thus can bedifferent for different digital facilities. In an aspect, the separationprocess separates devices having critical dimension greater/smaller thana predetermined value, for example, to be fabricated in differentfabrication facilities. For example, devices within the digital criticaldimension tolerance are separated to be fabricated in the digitalfacility. Alternatively, devices having critical dimension outside thetolerance of a digital facility are separated to be fabricated in aseparate facility. Operation 32 fabricates the separated digital andanalog portions in different fabrication facilities. Operation 33assembles the separately fabricated portions into a complete circuit. Inan aspect, the assembly utilizes a transfer printing process withphotosensitive thermally decomposable adhesive to handle small digitalcomponents.

FIG. 3B illustrates an alternative process flow for the presentfabrication separation process. Operation 31B separates an IC designinto different portions, for example, a digital portion and an analogportion, with the digital portion comprising components common to adigital fabrication facility and the analog portion comprisingcomponents not common to the digital fabrication facility. One of thecharacteristics of a digital facility is the ability to fabricate a chiproutinely, effectively and with low cost. Thus by separating thecomponents not common to the digital facility, the fabrication cost forthe digital portion can be minimized. After the design separation,operations 32 and 33 fabricate the portions in different fabricationfacilities before assembling into a complete circuit.

In an embodiment, the present separation process is based on the digitalfabrication facility. For example, the fabrication processes requiredfor making passive components are significantly different from thoseneeded for semiconductors. A MOS or CMOS (complementary metal oxidesemiconductor) transistor consists of electrodes (conductors),semiconductors (silicon with or without dopant), and insulators (almostexclusively silicon oxide), all patterned at very small sizes. Passivecomponents such as capacitors consist of conductors and insulators, butoften without small scale patterning, and ideally using insulators whichhave the highest possible dielectric constants and are not suitable fortransistors. Thus the fabrication processes are not easily meshed in asingle production line, and increased expense results. Even such aseemingly simple change as the addition of a Schottky diode (a metal ona semiconductor) is often not available in a CMOS foundry (because thedesired metal, for example, or perhaps the required doping level, is notpart of the CMOS process). In general, CMOS-based digital logic iscomprised of a very standardized, repetitive set of components (andhence fabrication processes), while analog circuitry may vary a greatdeal in its composition, size and arrangement according to the design.

FIG. 3C illustrates another alternative process flow for the presentseparation fabrication process. Operation 31B separates an IC designinto different portions, for example, a digital portion and an analogportion, with the analog portion comprising passive components havingdielectric different from silicon dioxide and/or silicon nitride, and/oractive components constructed in a non-MOS or non-CMOS process.Alternatively, the separation process brings to a separate facility allcomponents constructed in a non-MOS or non-CMOS process. Examples ofnon-CMOS process include analog devices, polysilicon/deposited orprinted single crystal silicon/amorphous silicon channel transistors,and/or power devices. After the design separation, operations 32 and 33fabricate the portions in different fabrication facilities beforeassembling into a complete circuit.

In an embodiment, the present invention discloses methods and integratedcircuits for fabricating a separated analog portion comprising passivecomponents utilizing dielectric materials different than silicondioxide, and active components utilizing channel materials differentthan single crystal silicon. The analog portion is then fabricated in aseparate analog fabrication facility, preferably different from adigital fabrication facility. In an aspect, the analog portion furthercomprises components not commonly processed in a digital fabricationfacility, components having materials not available in a digitalfabrication facility, components having critical dimensions greater or5× greater than the critical dimension of a digital fabricationfacility, or any combination thereof. In an aspect, the analog portionfurther comprises active components constructed in non-CMOS processes,or components to minimize the chip area of the digital portion. Also,the passive components of the analog portion can comprise capacitorstructures, or the dielectric material utilized in the passivecomponents has a dielectric constant greater than that of silicondioxide. The active components of the analog portion can comprise thinfilm transistors, or the channel material utilized in the activecomponents comprises one of polysilicon, amorphous silicon, anddeposited or printed single crystal silicon. In an aspect, singlecrystal silicon channel devices can be put in either digital or analogportions. For example, substrate single crystal silicon channel devices,such as channels fabricated from single crystal silicon wafer or SOI(silicon-on-insulator) wafer, are typically fabricated in a digitalfabrication facility. Polysilicon or amorphous silicon channel devicescan be separated into analog portions and fabricated in an analogfacility. Deposited or printed single crystal silicon channel (such asan area of ultrathin single crystal silicon is printed or deposited on asubstrate, and being used for a channel area) can be fabricated ineither digital or analog facility. In an aspect of the presentinvention, deposited or printed single crystal silicon channels areseparated into analog portions, and fabricated in an analog facility.

In an aspect, the analog and digital portions comprise bond pads for I/Oconnections between the portions for assembling to form the completeintegrated circuit. The assembling can comprise bonding correspondingbond pads between the analog and digital portions, wherein the portionsare placed in an at least partially stacked configuration, or in an atleast partially coplanar configuration.

In an embodiment, the present separation process and IC is applied toRFID tags. FIG. 4A illustrates an exemplary separation process for RFIDtag based on capacitors with high dielectric constant (i.e., dielectricconstant greater than that of silicon dioxide). Operation 41A separatesa RFID design into multiple portions, for example, a digital portion andan analog portion, with the analog portion comprising high dielectricconstant capacitors. After the design separation, operations 42 and 43fabricate the portions in different fabrication facilities beforeassembling into a complete circuit.

In a passive RFID tag, power must be supplied to the tag from theincident RF electromagnetic field. The field generates a high frequencyAC signal in the antenna, while the circuit must be powered by a DCconstant voltage source at the value which turns transistors on.Conversion is commonly accomplished by a diode-based rectifier circuit,which may be comprised of semiconductor diodes and capacitors. At leastone of these capacitors, and more generally more than one, is requiredto have a relatively high value in order to supply the required power tothe circuit. For example, in a simple diode rectifier a capacitancevalue of around 300 picofarads might be required to supply a power of 30microwatts to a circuit, starting from a 100 microwatt signal at 935 MHzreceived by the antenna from the reader. If the dielectric is siliconoxide (as commonly found in IC processes), and the thickness is 100 nm(which with IV across it is subject to a field of 0.1 MV/cm, which is areasonable margin of safety to avoid dielectric breakdown which couldoccur at field strengths of the order of 1 MV/cm or more), the arearequired would be 1 mm². Various circuit design strategies may beemployed to reduce this capacitance requirement, but areas which are asubstantial fraction of an mm² are still required.

Capacitor area can be reduced by increasing the dielectric constant ofthe insulator. Certain complex oxides, for example, are known to havefar higher dielectric constants. BaSrTiO2 has a value of about 500;other perovskites may attain values of several thousand. These materialsare not part of standard CMOS processing lines and cannot be providedintegrated with CMOS transistors. However, devices using them can bereadily made in thin film form by sputtering and evaporation processeswhich are well known in the art, so they can be made in a secondcomponent and integrated with silicon diodes. In a typical UHF RFIDcircuit, as described, for example, by Rob Glidden, et al., in the IEEECommunications Magazine, pp. 140-151, August 2004 (which refers to achip sold by Impinj, Inc., of Seattle, Wash.), hereby incorporated byreference, the total number of capacitors is 2533 (with about 39,000transistors). Many of these capacitors are small (associated with solidstate memory cells, for example), but many are not. Glidden, et al.,show a photograph of their chip in which there is a block of “Storagecapacitors” which occupies roughly 7% of the area. The “Rectifier”blocks (of which there are two) occupy a similar area.

According to the present invention, at least a block of storagecapacitors, as well as the capacitors in the rectifier section, would beformed on a separate substrate, whose processing cost would be far lessthan that of CMOS. The exact cost of any thin film process depends ofcourse on the details, but a rough estimate can be obtained by notingthat the cost of making thin film amorphous silicon transistors is inthe vicinity of 4¢/cm². Since such transistors are comprised of 4deposition and masking layers, with lithography in the 1-2 μm range, thecost may be of the same order. Clearly there are major savings (on a perunit area basis) to be obtained from forming such components ascapacitors and diodes in a separate layer which does not experience thecomplexity of high resolution CMOS processing. It is to be understoodthat these savings result in part because lower resolution lithographyis used, in part because there are fewer and less demanding processsteps, and in part because a less expensive substrate is used. Only whenthe characteristics of the product are not impaired by these aspects(e.g., lower patterning resolution, less precise control of smallimpurity concentrations, etc.) and the resulting area is stillacceptably small can one avail oneself of the lower area cost process.This is very strongly the case for capacitors, and to a lesser butsignificant case for diodes.

In an embodiment, the present invention discloses RFID circuits andmethods for fabricating an RFID circuit with a separate analog portioncomprising a capacitor utilizing dielectric material having dielectricconstant greater than that of silicon dioxide. The analog portion isthen fabricated in a separate analog fabrication facility beforeassembling to form a complete RFID circuit with the digital portion. Inan aspect, the analog and digital portions comprise bond pads for I/Oconnections between the portions for assembling to form the completeRFID circuit. The assembling can comprise bonding corresponding bondpads between the analog and digital portions, wherein the portions areplaced in an at least partially stacked configuration, or in an at leastpartially coplanar configuration.

In an embodiment, the present separation process is applied to singlecrystal silicon or polysilicon in RFID tags, or to substrate ordeposited/printed single crystal silicon channel in RFID tags. FIG. 4Billustrates an exemplary separation process for RFID tag based on singlecrystal silicon or polysilicon fabrication process. Operation 41Bseparates a RFID design into multiple portions, for example, a digitalportion and an analog portion, with the analog portion comprisingpolysilicon channel transistors or any active devices having channeldifferent than single crystal silicon. Alternatively, the digitalportion comprises devices having single crystal silicon channel. Afterthe design separation, operations 42 and 43 fabricate the portions indifferent fabrication facilities before assembling into a completecircuit. The separation between substrate single crystal silicon anddeposited/printed single crystal silicon can be similarly constructed.

The RFID analog circuitry also requires transistors, as noted above, andthese transistors, if made using conventional CMOS processes, cannot beshrunk according to Moore's Law due to the requirements to handlecertain values of current, voltage or power. However, when thetransistors are already so large, for example with gates of severalmicrometers width and length, they may be advantageously made usingother processes. In particular, they may be fabricated frompolycrystalline silicon (generally abbreviated “polysilicon”), which istypically formed by depositing amorphous silicon (by chemical vapordeposition, for example) and then re-crystallizing the amorphous layerusing thermal or laser heating sources. This process may be carried outon glass substrates which are quite large (several square meters) athigh throughput, and is currently used to make drive electronics forflat panel displays. The cost of the transistors is somewhat higher thanfor amorphous silicon transistors, due to the recrystallization step,but not by a large factor, while the electrical performance is farbetter. The electron mobility, for example, is often reported to be inthe range of about 120-300 cm2/Vsec, which is within a factor of 2-5 ofthat found in crystalline silicon. For example, Sharp Corporationreports a mobility 600× larger than that in amorphous silicon, which isknown to be generally around 1 cm²/Vsec.

The lateral size of the analog circuit transistors in RFID tags isrelatively large, yet the number of them is quite small compared todigital logic (in many designs less than about 100 total, and in somecases less than 50). Thus these devices may be advantageously made usinga polysilicon process even though they may need to be still larger(since the series resistance of the device, or its current carryingcapacity, is related to the mobility). The cost may be, for example,more than the cost of amorphous silicon transistors, but not so muchmore that the resulting circuits are not competitive for use in flatpanel displays for televisions or consumer electronics. Thus, even if(as an illustrative example) the cost is 1/50 of that of CMOS per unitarea instead of 1/100 (as for amorphous silicon), while the arearequired is 2-5 times as much, the savings are about 10× to 25×. Thearea process cost of CMOS is not decreasing substantially (if at all)with time, while the area process cost of polysilicon thin filmtransistors has been decreasing (due to the introduction of larger areaprocess equipment and larger volumes in the flat panel display industry,as well as improvements in the technology of laser annealing); hencethis advantage is a robust one and will if anything improve with time.

In addition to polysilicon channel devices, other channel materialtechnologies may be used in the scope of the invention. For example,GaAs or other III-V transistors (i.e., made using elements from GroupsIII and V of the periodic table) may be attractive for certain aspectsof the analog function of an RFID chip. These devices could notpreviously be used because it has not been possible to combine III-Vmaterials of device quality with silicon on a single substrate, and itis not generally cost-effective to place different chips on a singleRFID tag substrate and connect them. However, with the present inventionthe two types of device technology may be combined effectively. Othertransistor technologies which might be used include GaN and relatedmaterials, ZnO, and SiGe. This list is not exclusive and does notexhaust the scope of the invention.

In an embodiment, the present invention discloses RFID circuits andmethods for fabricating an RFID circuit with a separate analog portioncomprising a transistor having polysilicon, amorphous silicon, depositedor printed single crystal silicon channel. The analog portion is thenfabricated in a separate analog fabrication facility before assemblingto form a complete RFID circuit with the digital portion. In an aspect,the analog portion further comprises thin film transistors, or III-V orII-VI channel devices. In an aspect, the analog and digital portionscomprise bond pads for I/O connections between the portions forassembling to form the complete RFID circuit. The assembling cancomprise bonding corresponding bond pads between the analog and digitalportions, wherein the portions are placed in an at least partiallystacked configuration, or in an at least partially coplanarconfiguration.

The area that may be put into the second substrate depends on the designdetails of the RFID tag. Frequently, however, in the prior art, thedigital portion of the circuit may comprise about 40-60% of the totalarea, and the analog portion 40-60%. Thus, a superior device may befabricated by shrinking the digital portion according to the availableCMOS technology, and shifting the analog portion to alternativesubstrates and technology. Depending on which alternative technology isused, and what the component sizes are, one or the other of these may bethe larger. If, for example, the digital portion is 50% of the area of achip which is 700 micrometers square, using 0.25 micrometer technology,implementation in 90 nm technology would result in a shrink from 0.00245cm² to 13% of that value, leading to a nominal cost of slightly over0.1¢ instead of 1¢. For the analog portion, a CMOS chip area of 0.00245cm² costing nominally 1¢ may be replaced by a polysilicon area of lessthan 2-5× greater (less because of the reduction in capacitor area), orless than (700 μm)² to (1100 μm)². Since the cost of this processing isexpected to be not more than 8¢/cm², the cost of the analog portion isreduced to about 0.04¢ to 0.1¢, which is similar to the digital cost andrepresents a 10-fold reduction in overall chip cost. It is to beunderstood that these numbers are for illustration only and do not limitthe scope of the invention, but serve to illustrate its value.

In an embodiment, the present invention discloses integrated circuits,such as RFID devices, and methods to form complete integrated circuits,by separating the IC design into multiple common-utilized componentsbefore assembling them into the complete circuit. In an aspect, theseparation process is based on minimum area of the components, meaningthe components are fabricated as small as possible. In an aspect, thelateral size of the components is 1 mm or less, and preferably 0.1-0.5mm or less. The thickness of the components is 100 um or less, andpreferably 50 um or less. Also the components are placed less than 100um, and preferably within a few microns or tens of microns, of eachother. In an aspect, the size of the present separation components arelimited by the bond pads and the assembling of interconnect wires.Conventional bond pads provide 50-100 micrometer lateral size. Thepresent invention also offers bond pads size of a few micrometers, orless than 10-15 micrometers.

The small size of the present separation components requires innovativeassembly processes, such as a transfer printing process, sinceconventional pick and place machinery does not have adequate resolutionto handle this size. Such small circuits can save money, are flexibleand can fit into unique spaces.

The present components are preferably completed in wafer form withoutany packaging before assembling. Eliminating the packaging step of theindividual components allows smaller component size with lower cost. Inan aspect, the packaging process can be performed after the completionof component assembling.

FIG. 5A illustrates an exemplary process of separating an IC design intosmall components, and the process of assembling small components to forma complete circuit. Operation 71A separates an IC design into multipleseparate functional components with the functional components beingsmall (<1 mm, preferably <0.1-0.5 mm), thin (<100 um, preferably <50um), and/or assembled closed to each other (<100 um, preferably <10-15um). Operation 72 fabricates the multiple components separately.Operation 73 assembles the multiple components into a complete circuit.In an aspect, the present invention discloses the process of assemblingsmall functional components into complete circuit, preferably by atransfer printing process utilizing photosensitive thermallydecomposable adhesive. In an aspect, the multiple functional componentsare assembled to form a complete integrated circuit, with the assemblingprocess comprising a thermal adhesion transfer process, or bondingcorresponding bond pads between the multiple functional components. Themultiple functional components can be assembled in an at least partiallystacked configuration or an at least partially coplanar configuration.

FIG. 5B illustrates an exemplary process of separating a RFID tagdesign. Operation 71B separates the RFID tag design into multiplefunctional components: a memory component, a digital logic component,and a RF transponder component. Operations 72 and 73 fabricate andassemble the multiple functional components into a complete RFID tag.

The multiple functional components can be assembled in a stackedconfiguration, in an at least partially stacked configuration, in acoplanar configuration, or in an at least partially coplanarconfiguration. The invention is not limited to placing only one chip ontop of the larger bottom one: two or more may be placed in this manner.In general they will not overlap each other; however it is possible forthere to be more than two layers. If there are only two layers,connection may conveniently be made between the two (or more) chips bysuch means as metal to metal bonding, as is marketed, for example, byDatacon Technology GmbH, Radfeld, Austria, under the name of AdvancedChip-to-Wafer-Technology. Alternatively, anisotropic conductiveadhesives which are well known in the electronic packaging art may beused. Isotropic conductive adhesives may be used effectively by printingthem on at least one chip of the pair over the connector pads. Yetanother alternative uses non-conductive adhesives which contract oncuring, exerting a force which pulls the chips together and makeselectrical contact between pads on the two chips due to mechanicalforce. Conventional flip chip techniques using solider bumps may also beused, although this results in wider spacing between the upper and lowercomponents, and may require underfill adhesives. Underfill adhesives maybe required even if the spacing is smaller (as with isotropic conductiveadhesives, for example). In all of these cases, the process isfacilitated by the fact that typically only a few conducting connectionsbetween the pair of chips are required; for example in the case of aseparate power supply chip (e.g. rectifier), only two connections areneeded.

In an embodiment, the present invention discloses RFID devices andmethods for fabricating an RFID circuit, comprising separating the RFIDcircuit into a digital logic component, a memory component and an RFcomponent, and then fabricating the multiple components separately. Thecomponents have bond pads for connection with each other, and thecomponents, when ready for assembling into the complete RFID circuit,have a lateral dimension less than 1 mm and a thickness less than 100microns. In an aspect, the components do not require packaging, and canbe assembled in wafer form through bond pad connections. In an aspect,the present invention discloses the process of assembling smallfunctional components into a complete circuit, preferably by a transferprinting process utilizing photosensitive thermally decomposableadhesive. In an aspect, the multiple functional components are assembledto form a complete integrated circuit, with the assembling processcomprising a thermal adhesion transfer process, or bonding correspondingbond pads between the multiple functional components. The multiplefunctional components can be assembled in an at least partially stackedconfiguration or an at least partially coplanar configuration.

One of the problems faced by conventional flip chip processes is thelack of complete planarity and flatness of the two surfaces which arebeing connected, especially since the lateral size of the chip is oftenof the order of centimeters (for example, 0.5 to 3 cm), and when such achip is placed on a conventional printed wiring board (made of variouspolymer and polymer composite laminates), some parts of the chip will betouching the board, while other parts may be many μms or tens of μmsaway. The connection process must nevertheless make uniform connectionsacross the entire chip. In addition, there is no mechanical access tothe bonding region after the package has been assembled and made readyfor bonding.

In the present invention, the use of dice which are much smaller thanthe prior art is enabled. When the lateral dimension is only a fewhundred micrometers or less, the uniformity of contact of two dice faceto face is much greater. When both surfaces are on silicon dice comingfrom a processed wafer, the flatness and uniformity of contact is stillgreater. Thus, flip chip connection technologies can be used whichrequire smaller spacing than could otherwise be used. Likewise, face-upassembly and connection technologies results in more mechanicallystable, flexible assemblies.

In an embodiment, the present invention discloses assembly structuresand processes to place chips with their faces upward. The presentinvention further addresses the bond pad requirements for futureproducts. Conventional flip chip bond pad periodicity is usually atleast 50 μm or more. Printing techniques, such as inkjet printing, whichmay be used for depositing dots of conductive adhesive or molten solder,cannot easily reach resolutions below about 20 μm. If there are only twoI/O connections in a chip 100 μm on a side, this is an acceptable bondpad size. However, if there are several I/O pads, they would soon occupyall the available area and more, and a flip-chip process would not befeasible. Thus, in an embodiment, the present invention discloses chipto chip interconnect techniques for stacked chips which use less area;preferably these techniques can make use of pads as small as 5 μm oreven less, preferably down to 2-3 μm.

In an embodiment, through-silicon via (TSV) technology is rapidly beingdeveloped and is now commercially available. It generally involvesdrilling vias by some means (mechanical or laser, for example), and thenplating the via with Cu. These vias may be relatively narrow, but areconstrained by aspect ratio. For example, if the thickness of thesubstrate is 25 μm, a 5 μm diameter represents a 5:1 aspect ratio, whichis not easy to fill with uniform metal deposits by electrodeposition. Inaddition, electrodeposition is time-consuming and not well suited tohigh-speed roll-to-roll processing.

When one has a small number of connections through a thin die, theconducting paths may advantageously be formed by filling the vias withvarious printable conductive materials, for example sinterable metalparticles, decomposable conductor precursor inks, or metal mixtureswhich, when fully alloyed, have a melting point (and mechanicalstrength) which is greater than that of the lower-melting component.Examples of the last class of materials are mixtures of Sn (m.p. 232 C)or Ga (m.p. 30 C) with higher melting point metals such as Cu. Thesemixtures may be sintered by a focused laser beam to minimize heating ofthe rest of the chips, or they may be sintered by purely thermal sourcesif the components can withstand this temperature for a short time aswould be used in a typical soldering process. Liquid metal with amelting point up to about 230 C may also be dispensed from heated inkjetprintheads such as are sold by Microfab, of Plano, Tex. The via diametermust be large enough to match the minimum droplet size from inkjetprinters, which is about 20 micrometers. For chips with small numbers ofI/O connections, this condition will be met.

In an embodiment, the present invention discloses assembly structuresand processes for higher density I/O pads. One preferred approachdeposits conductor lines which pass over the edge of the upper chip andmake contact to pads on the lower chip. In conventional packaging,wirebonding is used in this manner. Wirebonds may be used in the presentinvention as long as there are not too many of them, since they aretime-consuming and comparatively expensive to form. For example, acommercially available wirebonder sold by Kulicke & Soffa, of WillowGrove, Pa., can make about 12-16 connections per second. Wirebonders canhandle wires with diameters of 25 μm or less.

FIGS. 6A and 6B illustrate cross section and top views of an embodimentof conduction lines 83 connecting a top chip 80 to a bottom chip 81.Both chips have their face upward, showing bond pads 82A and 82B on theupper surfaces of the chips. Lines of only a few μm in width can beconveniently formed by aerosol printing, using for example a printersold by Optomec, Inc., of Albuquerque, N. Mex. They may also be formedby laser processing, as disclosed, for example, in co-pendingapplications assigned to Terepac Corp. If such lines are written fromsmall pads (of a few micrometer lateral extent) to corresponding pads onthe lower chip, many I/O connections can be formed even in a 100 μmchip.

One problem with writing such lines over the edge of a chip is thatelectrical discontinuity may be caused by the sharp chip edge 84, andthe sidewall may be incompletely covered. In an embodiment, the presentinvention discloses some angle forming a gradual transition at thesidewall of the chips. One way to achieve such an angle is to use laserablation, forming a gradual slope or ramp (over at least a few μms) fromthe surface of the top chip to that of the bottom. Such a laser ablationprocess is facilitated by making the top chip relatively thin, forexample a few tens of μms or less. After the formation of gradualslopes, electrical connection can be formed by a transfer printingprocess, such as that of U.S. Pat. Nos. 6,946,178 and 7,141,348.

FIGS. 7A and 7B illustrate exemplary configurations of gradual slopes84A on the top chip 80. In FIG. 7A, one slope 84A is formed on the topchip 80, allowing a gradual support for the electrical connection 83between the bond pad 82A of the top chip 80 to the bond pad 82B of thebottom chip 81. FIG. 7B illustrates chip 80 with two gradual slopes 84A,supporting multiple sides of electrical interconnects 83.

FIG. 8A illustrates an exemplary structure and process of forming thegradual slope 88A on top chip 80. One side edge of the chip 80 is cutalong the edge to allow the gradual transition for a plurality ofinterconnects 83 connecting the bond pads of the top chip to the bottomchip. Other edges can also be cut (not shown). A laser beam can passalong the edge of the top chip 80 to ablate the material to form thesloped edge 88A.

FIG. 8B illustrates another exemplary structure and process of formingthe gradual slope 88B on top chip 80. The side edge of the chip 80 iscut along the direction of the interconnect 83 to provide a gradualslope for the interconnect. One or more interconnects 83 can beprovided. A laser beam can pass along the direction of the bond padconnection, typically perpendicular to the chip edge, ablating thematerial to form the gradual slope.

FIG. 9A illustrates an exemplary flowchart for a formation of the slopeto facilitate the connection between the chips. Operation 110 positionsa top component (or chip) on top another component (or chip). Thecomponents have bond pads on the top surface, to be connected byinterconnect wires. The top component is preferably positioned to alignthe bond pads between the top and bottom components, for example, thebond pads of the top component are arranged to be parallel to thecorresponding bond pads of the bottom component. Operation 111A formsgradual slope along an edge of the top component, preferably along anedge that is closest to the bond pads. In an aspect, the slope formationis accomplished by a laser ablation process, running a laser along theedge of the top component. Operation 112 forms interconnect wiresconnecting the bond pads of the top component to the bond pads of thebottom component.

FIG. 9B illustrates an exemplary flowchart for a formation of the slopeto facilitate the connection between the chips. After positioning a topcomponent on top of a bottom component (operation 110), operation 111Bforms a gradual slope along the direction connecting the bond pads, fromthe bond pad on the top component to the corresponding bond pad on thebottom component. The slope line is typically perpendicular to the edgeof the top component, since the bond pads of the top component arenormally arranged along the edge. The width of the slope line is wideenough to accommodate a predetermined number of interconnect wires. Inan aspect, the slope formation is accomplished by a laser ablationprocess, for example, running a laser along the direction of theinterconnect wires, perpendicular to the edge of the top component.Multiple laser passes can be performed to achieve a proper width, forexample, to allow positioning a number of interconnect wires. After theformation of the gradual slope, interconnect wires are formed to connectthe bond pads (operation 112).

In an embodiment, the present invention discloses methods and structuresfor interconnecting bond pads between integrated components, comprisingforming a beveled slope of a top integrated component on a bottomintegrated component. An interconnect wire is placed over the beveledslope, linking a top bond pad of the top integrated component to abottom bond pad of the integrated component. The interconnect wire isthen welded to the bond pads. The component can have one or more beveledslopes. In an aspect, the top integrated component is positioned on thebottom integrated component before forming the beveled slope. In anotheraspect, the top integrated component is positioned on the bottomintegrated component after forming the beveled slope. The beveled slopecan be formed by laser ablating the edge of the top integratedcomponent, along the edge of the top integrated component. The beveledslope can formed by laser ablating from the top bond pad to the bottombond pad, running from the top bond pad to the bottom bond pad. Thepresent beveled slope can facilitate placing interconnect wires over theedge of the top integrated component. The laser ablation processtypically comprises a laser providing concentrated energy to shape theintegrated components, for example, by evaporating and/or bombarding thematerial under the laser focus points. Other methods for beveling can beutilized, such as water-jet guide laser.

In an aspect, placing interconnect wires comprises printing interconnectlines, where the beveled slope allows printing of interconnect linesover the edge of the top integrated component. In another aspect,multiple interconnect wires are placed over the beveled slopesimultaneously. Also multiple interconnect wires are welded to the bondpads of an integrated component simultaneously.

In an embodiment, the present invention discloses a beveled slopeintegrated component for facilitating forming interconnect wires betweenintegrated components comprising a beveled slope edge of the integratedcomponent in the vicinity of the bond pads, the beveled slope runningfrom the surface having the bond pad to the other surface of theintegrated component. The beveled slope edge can have a rounded corner.In an embodiment, the present invention discloses a chip assemblycomprising a bottom integrated component located under the beveled slopeintegrated component with one or more interconnect wires over thebeveled slope, linking the bond pad of the beveled slope integratedcomponent to the bottom bond pad.

The bonding process of the interconnect wires onto the bond pads can beeither ultrasonic or laser welding. In an embodiment, the presentinvention discloses a bonding process for dense interconnection, bondinga plurality of interconnect wires to a plurality of bond pads at a sametime. An optional separation process can be performed to separate thewires, to prevent any cross connection. For example, a line ofconductive adhesive can be formed along the bond pads before positioningthe interconnect wires to the bond pads. Alternatively, after the wiresare positioned on the bond pads, a laser beam may be run along thedirection of the bond pads, bonding the wires onto the bond pads.

FIG. 10 illustrate an exemplary process for simultaneously forming aplurality of interconnect wires. FIG. 10A shows a top chip 80 positionedon a bottom chip 81, with the bond pads 82A of the top chip aligned withthe bond pads 82B of the bottom chip. Gradual slope can be used tofacilitate the wiring connections. Alternatively, the bond pads betweenthe two chips can be positioned according to an assembly process, whichmight or might not be parallel as shown in this figure. Afterpositioning the two chips, a line of isotropic conductive adhesive 120may be printed at the ends of the wires, in a direction 121perpendicular to the long direction of the wires (FIG. 10B). Thisadhesive is designed to electrically connect the wire ends to theunderlying pads, but it also connects the pads to each other. Inaddition to isotropic conductive adhesive (e.g., commercially availableAg-filled epoxies or the like), any other form of printed conductor maybe used, such as printable nanoparticulate Ag or other metal precursorinks. The laser may be used to cure the ink. A plurality of wires 83 arepositioned on the corresponding bond pads, on the conductive adhesive(FIG. 10C). The connections between the bond pads are then separated(FIG. 10D), for example, along the direction 122 of the interconnectwires. In an aspect, a laser may be used to form cuts between the pads,thereby isolating them and leaving the desired connections only. Thesame laser used to cure the ink can be used to ablate the cuts (usingdifferent conditions between curing and cutting, for example, lowerpower for curing and higher power for ablating). This configurationillustrates the bonding process for a chip stacked configuration, butother configurations are possible, for example, side-by-side, partiallystacked, or partially co-planar. The process can be repeated for theother end of the connection wires, bonding one end after the completionof one end, or bonding two ends simultaneously. For example, theconductive adhesive can be applied to the bond pads of both chips,forming two adhesive lines, one for each chip. The wires can be placed,and both ends of the wires can be pushed to the adhesive to formcontact. Laser or ultrasonic bonding can be applied to both ends of thewires.

FIG. 11 illustrate another exemplary process for simultaneously forminga plurality of interconnect wires. FIG. 11A shows the positions of thetop chip 80 on the bottom chip 81. In FIG. 11B, a plurality ofinterconnect wires 83 are positioned on the corresponding bond padsbefore a bonding process, such as ultrasonic, bonds the plurality ofwires onto the bond pads, preferably simultaneously. For example, alaser beam can travels in the direction 126 along the bond pads, forminga bonding line 125, bonding the multiple interconnect wires 83 to thebond pads 82A (FIG. 11C). A separation process, such as a laser ablationcutting, can cut the connection 127 between the adjacent bond pads (FIG.11D). In an aspect, the laser beam might skip the spaces between thebond pads, for example, by a mirror reflection assembly, so that theadjacent bond pads are not electrically connected, and a separationprocess might not be needed. Other alternatives and options can beapplied.

FIG. 12A illustrates an exemplary flowchart for the bonding of multipleinterconnect wires to the corresponding bond pads. Operation 130 placesa conductive adhesive line along the multiple adjacent bond pads, forexample by a printing process. Optional laser curing process can beperformed for curing the printed ink. The conductive adhesive istypically continuous, connecting the adjacent bond pads along an edge ofthe chip. The conductive adhesive can be applied to the bond pads at oneor both ends of the interconnect wires. Operation 131 places a pluralityof interconnect wires onto the corresponding bond pads that already havethe conductive adhesive. An optional forcing element can be used to pushthe wires down into contact with the bond pads. Suitable forcingelements include rollers, pins and compressed air. One, more or all ofthe contacts along one side of a die can be pushed into place by thesame forcing element at once. The interconnect wires can make contactwith one row of bond pads at one end of the interconnect wires.Alternatively, the interconnect wires can make contact with both rows ofbond pads at both ends of the interconnect wires. Also, after theinterconnect wires make contact with one row of bond pads, the processcan go back to operation 130 to place another conductive adhesive lineonto the other row of bond pads on the other chip. The process cancontinue with operation 131 for placing the other end of theinterconnect wires onto this bond pads with adhesive. Operation 132separates the connected bond pads, for example, by laser ablating thespaces between the adjacent bond pads.

In an embodiment, the present process bonds the rows of bond pads insequence, meaning bonding one row of bond pads to one end of theinterconnect wires before starting on another row of bond pads to bondthe other end of the interconnect wires. For example, a line ofconductive adhesive is printed on a row of bond pads, then theinterconnect wires are placed on the adhesive at one end to bond theinterconnect wires to the corresponding bond pads. After separating theadjacent bond pads, for example, by a laser ablation process, anotherline of conductive adhesive is applied on another row of bond pads, andthe process is repeated for the other end of the interconnect wires.

In another embodiment, the present process bonds the rows of bond padsin parallel, meaning two rows of bond pads are processed together tobond both ends of the interconnect wires simultaneously. For example,two lines of conductive adhesive are printed on two opposite rows ofbond pads, then the interconnect wires are placed on the adhesive linesat both ends to bond the interconnect wires to the corresponding bondpads. A laser ablation process then separates the adjacent bond pads inboth rows. Other variations of the present process can be used, such asbonding one end of the interconnect wires to one row of bond pads,repeating the bonding process with the other end of the interconnectwires before laser ablation separating the adjacent bond pads on bothends of the interconnect wires.

FIG. 12B illustrates another exemplary flowchart for the bonding ofmultiple interconnect wires to the corresponding bond pads. Operation135 places a plurality of interconnect wires onto the corresponding bondpads. An optional forcing element can be used. Operation 136 bonds theinterconnect wires to the corresponding bond pads, for example, byultrasonic bonding or laser bonding. Depending on the bonding process,the adjacent bond pads may or may not be electrically connected. Anoptional separation process between the adjacent bond pads can beperformed to separate the electrical connections between adjacent bondpads. The separation process can be a laser ablation cutting along thelength of the interconnect wires, in the spaces between the adjacentbond pads.

The present multiple wire bonding process can be performed after thechips are positioned in place, for example, by a stacked configuration,a partial stacked configuration, a co-planar configuration, or a partialco-planar configuration. Alternatively, the present multiple wirebonding process can be performed on a single chip, forming connectionsto the adjacent bond pads of a chip, and leaving the other ends of theconnections to be connected at a later time.

Suitable materials for the wires include all good conductors, preferablywith superior ductility as well as high conductivity and amenable tomaking good welded connections. Preferred choices would include Ag, Cu,Al, Ag, and Sn, as well as alloys of these materials. The wires may becoated with other alloys for weldability. Metal filaments of a few μmdiameter are commercially available, as well as means of handling them.

In an embodiment, the present invention discloses systems and methodsfor interconnecting bond pads between integrated components, comprisingplacing and bonding multiple interconnect wires on corresponding bondpads of an integrated component, and then electrically separating theadjacent bond pads. The components can be assembled in an at leastpartially stacked configuration or an at least partially coplanarconfiguration. In an aspect, at least one of the components has abeveled slope. Also placing the multiple interconnect wires can compriseprinting the multiple interconnect wires, including simultaneouslyprinting the multiple interconnect wires. In an aspect, bonding themultiple interconnect wires to the bond pads comprises one of laserwelding and applying a conductive adhesive. Further, electricallyseparating the adjacent bond pads comprises laser ablating between theadjacent bond pads.

In an embodiment, the present invention discloses systems and methodsfor interconnecting bond pads between integrated components, comprisingfirst applying a conductive adhesive line along multiple bond pads of aintegrated component, then placing and bonding multiple interconnectwires on the conductive adhesive line corresponding to the bond padsbefore laser ablating between the adjacent bond pads to separate theinterconnect wires. In an aspect, applying a conductive adhesive linecomprises printing a conductive adhesive line. Further, the method cancomprise curing the conductive adhesive line, and pushing theinterconnect wires onto the bond pads. The components can be assembledin an at least partially stacked configuration or an at least partiallycoplanar configuration. In an aspect, at least one of the components hasa beveled slope. Also placing the multiple interconnect wires cancomprise printing the multiple interconnect wires, includingsimultaneously printing the multiple interconnect wires. In an aspect,bonding the multiple interconnect wires to the bond pads comprises oneof laser welding and applying a conductive adhesive. Further,electrically separating the adjacent bond pads comprises laser ablatingbetween the adjacent bond pads.

In an aspect, a conductive adhesive line is also applied along the bondpads of the another integrated component, then the multiple interconnectwires are bonded to the bond pads of the another component, before beinglaser ablated between the adjacent bond pads.

In an embodiment, the present invention discloses systems and methodsfor interconnecting bond pads between integrated components, comprisingfirst placing multiple interconnect wires on the corresponding bond padsand laser welding the multiple interconnect wires to the bond padsbefore laser ablating between the adjacent bond pads to separate theinterconnect wires. In an aspect, laser welding comprises laser ablatingalong the adjacent bond pads. The same laser can be used for welding andablating. Further, the method can comprise pushing the interconnectwires onto the bond pads to facilitate laser welding. The components canbe assembled in an at least partially stacked configuration or an atleast partially coplanar configuration. In an aspect, at least one ofthe components has a beveled slope. Also placing the multipleinterconnect wires can comprise printing the multiple interconnectwires, including simultaneously printing the multiple interconnectwires. In an aspect, the other ends of the multiple interconnect wiresare bonded to the other integrated component, either by a conductiveadhesive line or by laser welding. Laser ablation between the adjacentbond pads of the other component can be performed for separating thewires at the other integrated component.

In an embodiment, the present invention discloses a denser wire bondingprocess using equipment which is already commercially available, whileproviding a means to make denser connections than that equipment canmake. In an aspect, the present invention uses a commercial wirebonderto directly deposit wires with smaller diameter. The present inventionrecognizes that the currently available wirebonders are limited torelatively large wire sizes because of the bonding process and notbecause of the ability to feed smaller filaments of wire. Thus, thepresent invention discloses a separate bonding assembly, such as a lasercutting and/or a laser welding, for the smaller wires of a wirebonder.In an aspect, the mechanisms of a wirebonder (capillary, feedingmechanics, and positioning technology) may be used to extrude thedesired length of wire over the desired location, and then a separatelaser is used to cut the wire and allow it to fall into position. Thelaser (or a different laser) is then used to weld the two ends of thewire.

FIG. 13 illustrate an exemplary process flow for the combination ofwirebonder and laser cutting/welding. FIG. 13A illustrates a top chip 80positioned on a bottom chip 81 with the bond pads 82 facing upward. Chip80 has a gradual slope 84A, for example, to help in positioning andbonding the interconnect wire. Other configurations can be used, forexample, co-planar chips, or partially co-planer or partially stackedchip, or top chip with vertical edges. A wirebonder 190 is positionedand aligned on the bond pads, using smaller wire 191. FIG. 13B shows thewire 191 extruded and the wirebonder 190 traveled along the direction ofthe bond pads. After a desired length is reached, a laser cut isperformed and the wire falls on the chips, in the direction connectingthe bond pads (FIG. 13C). A laser welds the top bond 154A (FIG. 13D) andthe bottom bond 154B (FIG. 13E). Optional forcing elements can be usedto facilitate the bonding.

FIG. 14 illustrates an exemplary process flow for the combination ofwirebonder and laser cutting/welding. Operation 230 extrudes correctlengths of interconnect wires over desired locations of the bond pads. Awirebonder can be used to provide the wire extrusion with the diameterof the interconnect wires suitable for high density interconnect.Operation 231 cuts the wires with a laser, and operation 232 welds theends of the interconnect wires to the bond pads. A laser can be used toweld the bonds.

In an embodiment, the present invention discloses a method forinterconnecting bond pads between integrated components, comprisingextruding an interconnect wire over a desired location of bond pads, andlaser cutting a correct length of the interconnect wire for falling onthe corresponding bond pads. The method can further comprise pushing theinterconnect wire onto the bond pad, and laser welding the interconnectwire to the bond pads. A same laser can be used for cutting and welding.One of the integrated components can have a beveled slope edge. In anaspect, extruding the interconnect wire comprises a mechanical feeder,or a wirebonder wire feeder mechanism.

In an embodiment, the present invention discloses a printing process toprint interconnect wires, over sharp edges of the chip, or over gradualsloped edges. In an aspect, narrow wires can be formed and laid down ona substrate which has been precoated with a thermally decomposableadhesive, for example, the thermally decomposable adhesive disclosed inU.S. Pat. Nos. 6,946,178 and 7,141,348. The wires can then be cut, usingfor example a laser, into sections which are the correct length to formthe interconnects. The substrate can then be used as a printing plate toplace the sections of wire in the appropriate places so that each end ofthe wire is placed over a contact pad, one end on the upper chip and oneon the lower. A laser is used to weld the end on the upper chip to itscontact pad. A forcing element may then be used to push the wire downinto contact with the pin. Suitable forcing elements include rollers,pins and compressed air. Then, the laser is used to weld the secondcontact. If several contacts need to be made, all of the contacts alongone side of a die can be pushed into place by the same forcing elementat once. Only a small pressure is needed, since the element does notneed to cause bonding, but only provide proximity so that welding can becarried out. If the welding process forms a liquid of at least one ofthe components (and somewhat softens the metal of the wire close to thejoint), then contact is facilitated by surface tension and gravity. Inan aspect, the pressure forcing element may be replaced by the use of alaser-formed ramp, as described in connection with the aerosol-writtenlines. If the slope is chosen correctly, the wire will lie on the chipwith each end supported by a bond pad, ready for welding.

In an embodiment, the wires on the thermally decomposable adhesive canbe metal lines, which are formed by deposition of metal films onto thethermally decomposable adhesive followed by patterning into lines. Thismethod could make narrower lines and make available a broader choice ofmaterials. Alternatively, a commercial wirebonder may also be used toform the desired lengths of metal wire. A wirebonder uses an electricalpulse to cut a thin wire, which is fed through a narrow diameter nozzle,at the desired point. The wirebonder provides the wires at the desiredlengths on the thermally decomposable adhesive, which then uses thethermally decomposable polymer transfer printing process and itsassociated printing tool to accomplish the alignment and placement ofthe wires. Only a part of the conventional wirebonding tool is used toprovide proper wire lengths, approximately aligned in the desireddirection.

FIG. 15 illustrate an exemplary printing process for interconnect wires.FIG. 15A shows a printhead 150 comprising a thermally decomposable layer151 where a plurality of interconnect wires 153 are attached. Thermallydecomposable layer 151 can comprise an adhesive polymer which attachesto the wires 153. The thermally decomposable adhesive 151 can lose theadhesion under high temperature and/or light, and release the wires 153.In FIG. 15B, the printhead 150 is brought to the proper place, with thewire 153 aligned to the bond pads 82 of the top chip 80 and the bottomchip 81. Alternatively, other configurations can be employed, such asco-planar chips, or partially co-planar or partially stacked. Also, thetop chip is shown with a gradual slope edge 84A to facilitate the wirebonding, but other chip edges can be used, such as vertical edges. InFIG. 15C, the decomposable layer 151 is released to place the wire 153on the bond pads 82. The gradual slope 84A can facilitate the placementof the wire 153. Laser welding at the top bond pad 154A (FIG. 15D) andat the bottom bond pad 154B (FIG. 15E) bonds the wire 153 to the bondpads. An optional forcing element can be used to help the bondingprocess.

FIG. 16 illustrate an exemplary process of preparing the wires withcorrect lengths on the printhead 150 coated with a thermallydecomposable adhesive 151. A plurality of wires is positioned on thethermally decomposable adhesive 151 on the printhead 150 (FIG. 16A), Acutting process, such as a laser cut 174, is performed to cut the wires173 into wires with correct length 173A (FIG. 16B). FIG. 16C shows theprinthead 150 having the correct length wires 173A ready for printing onthe chip bond pads (FIG. 16C).

FIG. 17 illustrate another exemplary process of preparing interconnectwires with correct lengths on the printhead 150 coated with a thermallydecomposable adhesive 151. A metal layer 180 is deposited on thethermally decomposable adhesive 151 (FIG. 17A), and patterned, forexample, with photoresist 185 (FIG. 17B), to form metal lines 183 ofcorrect lengths and orientations (FIG. 17C). The photoresist 185 isstripped and the printhead 150 is ready for printing the interconnectwires 183 on chips (FIG. 17D).

FIG. 18 illustrates another exemplary process of preparing interconnectwires with correct lengths on the printhead 150 coated with a thermallydecomposable adhesive 151. A wirebonder 190 provides wire 191 near thethermally decomposable layer 151 (FIG. 18A). The diameter of the wire191 is determined by the desired printing process, and can be smallerthan the diameter used in a wirebonder process. After the correct lengthis reached (FIG. 18B), the wire is cut, for example by a laser cutprocess (FIG. 18C). After a plurality of wires with correct lengths arearranged on the thermally decomposable layer 151 (FIG. 18D), theprinthead is ready for printing wires on the chips (FIG. 18E).

FIG. 19 illustrates an exemplary flowchart for the present printingprocess of interconnect wires. Operation 210 forms interconnect wireswith correct lengths on a thermally decomposable adhesive on a printheadsubstrate. The printhead then moves to the proper locations, positioningthe interconnect wires on corresponding bond pads (operation 211). Thethermally decomposable adhesive is then released to allow theinterconnect wires to fall on the bond pads (operation 212). Thereleasable adhesive can be released by heat, light or a combinationthereof. An optional pushing mechanism can be performed to push the endsof the interconnect wires against the bond pads (operation 213). Awelding process such as a laser welding or ultrasonic welding is thenperformed to weld the interconnect wires onto the bond pads (operation214).

FIG. 20A illustrates an exemplary process of forming wires with correctlengths on the thermally decomposable layer on the printhead. Afterforming a plurality of interconnect wires on the thermally decomposablelayer (operation 220), a laser is applied to cut the interconnect wiresto the correct lengths (operation 221). FIG. 20B illustrates anotherexemplary process of forming wires with correct lengths on the thermallydecomposable layer on the printhead. After depositing a layer of metalon the thermally decomposable layer (operation 225), a patterningprocess is applied to form interconnect wires with correct lengths(operation 226). The patterning process can use photolithographyprocess, with photoresist mask and wet or dry etching steps. FIG. 20Cillustrates another exemplary process of forming wires with correctlengths on the thermally decomposable layer on the printhead. After awirebonder delivers interconnect wires on the thermally decomposablelayer (operation 228), a laser is applied to cut the interconnect wiresto the correct lengths (operation 229), which are then laid down on thethermally decomposable layer.

In an embodiment, the present invention discloses methods for printinginterconnect wires on bond pads between integrated components,comprising forming interconnect wires on a thermally decomposableadhesive on a printhead substrate, then positioning the printhead sothat the interconnect wire is located over a desired location of bondpads, and releasing the interconnect wire to fall on the correspondingbond pads. In an aspect, one of the integrated components has a beveledslope to facilitate the printing process. In addition, a pushing elementcan be included to help pushing the interconnect wire onto the bond pad.A laser can be included to weld the interconnect wire to the bond pads.Alternatively, a conductive adhesive can be applied on the bond padsbefore releasing the interconnect wire. The printhead can simultaneouslyrelease multiple interconnect wires over the corresponding bond pads.The integrated components can be positioned in an at least partiallystacked configuration, or an at least partially coplanar configuration.

In an embodiment, the present invention discloses a method for forminginterconnect wires on a printhead, comprising placing interconnect wireson a thermally decomposable adhesive on the printhead substrate, andthen laser cutting the interconnect wires to correct lengths. In anaspect, the interconnect wires are separated by a same distance as theseparation of adjacent bond pads, and multiple interconnect wires arecut to a same length for simultaneously printing on multiple bond pads.Also, a layer of thermally decomposable adhesive can be coated on theprinthead substrate. Alternatively, the layer of thermally decomposableadhesive on the printhead substrate is transferred to the printheadsubstrate. The thermally decomposable adhesive is releasable by at leastone of heat and light.

In an embodiment, the present invention discloses a method for forminginterconnect wires on a printhead, comprising depositing a conductivelayer on a thermally decomposable adhesive on the printhead substrate,and then patterning the conductive layer to form interconnect wires withcorrect lengths. In an aspect, patterning the conductive layer comprisesa photolithography process. The interconnect wires are separated by asame distance as the separation of adjacent bond pads, and multipleinterconnect wires are cut to a same length for simultaneously printingon multiple bond pads. Also, a layer of thermally decomposable adhesivecan be coated on the printhead substrate. Alternatively, the layer ofthermally decomposable adhesive on the printhead substrate istransferred to the printhead substrate. The thermally decomposableadhesive is releasable by at least one of heat and light.

In an embodiment, the present invention discloses a method for forminginterconnect wires on a printhead, comprising extruding an interconnectwire on a thermally decomposable adhesive on the printhead substrate,and then laser cutting a correct length of the interconnect wire to fallon the thermally decomposable adhesive. In an aspect, extruding theinterconnect wire comprises a mechanical feeder or a wirebonder wirefeeder mechanism. The interconnect wires are separated by a samedistance as the separation of adjacent bond pads, and multipleinterconnect wires are cut to a same length for simultaneously printingon multiple bond pads. Also, a layer of thermally decomposable adhesivecan be coated on the printhead substrate. Alternatively, the layer ofthermally decomposable adhesive on the printhead substrate istransferred to the printhead substrate. The thermally decomposableadhesive is releasable by at least one of heat and light.

In an embodiment, the present invention discloses a printhead forprinting interconnect wires, comprising a thermally decomposableadhesive on a printhead substrate, a plurality of interconnect wirespatterned to the correct lengths disposed on the thermally decomposableadhesive, and a release mechanism for releasing the thermallydecomposable adhesive. The interconnect wires are separated by a samedistance as the separation of adjacent bond pads, and multipleinterconnect wires are cut to a same length for simultaneously printingon multiple bond pads. The thermally decomposable adhesive is releasableby at least one of heat and light. In an aspect, the interconnect wiresis placed on the thermally decomposable adhesive and laser cut to thecorrect lengths. Alternatively, the interconnect wire is patterned tothe correct lengths from a deposited conductive layer on the thermaldecomposable adhesive.

In an embodiment, the present invention discloses a higher density(smaller pad pitch) interconnect wire bonding process, where theconnection may be made by a metal ribbon instead of wire; i.e., a wirewhich is substantially wider than its thickness. This ribbon is bondedsimultaneously to the pads along one side of a die, using eitherconventional ultrasonic bonding or laser welding, or a combinationthereof. The other end is similarly bonded to the pads on the lower die.A laser is then used to ablatively cut along the length of the ribbon,thereby separating it into individual parallel wires. An advantage ofthis technique would be higher throughput, since the tool only has toperform one alignment operation to get the ribbon into the proper placefor bonding many pads. Subsequently all of the bonds may be formed in asingle ultrasonic welding operation, or they may be formed by a laserwhich addresses each pad in sequence but does not need mechanical motionto do so (apart from the motion of a beam steering unit). The resultingwires may be encapsulated for protection and stability by applying adroplet of curable adhesive. The width of the wires formed in this wayis limited only by the resolution of laser cutting, which can readily bea few μm.

In a preferred embodiment, the ribbon described previously may also beplaced using the thermally decomposable polymer printing process. Theribbon may optionally be treated by a droplet of nonconductive orconductive adhesive to help hold it in place until welding is finished.The placement of the ribbon may also be facilitated by using a laser toform a slope at the edge of the die (similarly to the ramps that havebeen described, but uniformly across the length or width of the die).

FIGS. 21 illustrate an exemplary process sequence for the present ribboninterconnect with laser welding. FIG. 21A shows a chip 80 positioned ontop of a chip 81 with the bond pads 82A aligned with the bond pads 82B.This figure shows a stacked configuration where chip 80 is stacked ontop of chip 81, but other chip configurations can be used, such asco-planar, partially co-planar, or partially stacked. In addition, thisfigure shows chip 80 having vertical edges, but other structures can beused, such as a chip having one or more gradual sloped edges. Further,this figure shows a configuration for bonding chips 80 and 81, but thepresent ribbon interconnect process can be used to bond to bond pads inone chip, with the other end of the ribbon interconnect to be configuredat a later time, for similar or different bonding processes or purposes.A ribbon interconnect 240 is positioned on the bond pads, connecting thebond pads of both chips (FIG. 21B). Then a bonding process, such asultrasonic welding or laser welding, can be used to bond the ribboninterconnect to the bond pads. FIG. 21C shows a laser welding process,welding the ribbon interconnect 240 to the individual bond pads 241 and241A of chips 80 and 81. After the bonding process, the ribboninterconnect is separated into individual interconnect wires, forexample, by a laser cutting process 242 along the length of the ribboninterconnect and between the adjacent bond pads.

FIGS. 22 illustrate another exemplary sequence, where the laser weldingruns continuously to bond and connect the adjacent bond pads. FIG. 22Ashows a chip 80 having bond pads 82A in the vicinity of a gradual slopeedge. Ribbon interconnect 240 is placed on the bond pads (FIG. 22B), anda continuous welding process bonds all the bond pads to the ribboninterconnect. For example, a laser beam travels in the direct 261 alongthe adjacent bond pads, welding the ribbon interconnect to the bond pads260 (FIG. 22C). A separation process, such as a laser cutting 252,separates the ribbon interconnect into individual wires (FIG. 22D).

FIGS. 23 illustrate an exemplary process sequence for the present ribboninterconnect with conductive adhesive. FIG. 23A shows a chip 80 havingbond pads 82A in the vicinity of a gradual slope edge. A line 250 ofconductive adhesive is placed on the bond pads, for example, by aprinting process along the adjacent bond pads 251 (FIG. 23B). Ribboninterconnect 240 is placed on the adhesive (FIG. 23C) to bond the withthe bond pads. An optional curing process can be performed. A separationprocess, such as a laser cutting 252, separates the ribbon interconnectinto individual wires (FIG. 23D).

FIG. 24 illustrates an exemplary flowchart for the present ribboninterconnect. Operation 270 positions a ribbon interconnect on aplurality of bond pads. After the ribbon interconnect bonds with thebond pads, operation 271 separates the ribbon interconnect intoindividual interconnect wires. The bonding process can be ultrasonicwelding or laser welding. The bonding process can be simultaneousbonding, bonding multiple bond pads together. The bonding process can besequential bonding, bonding individual bond pads separately. Theseparation process is preferably a laser cutting process, providing thedesired high density interconnection. In an aspect, each individual wireconnects to a bond pad, forming electrical connection between two bondpads. Other configurations can be used, such as an interconnect wire canbe split with one end connected to two connected bond pads and the otherend connected to two separate bond pads.

FIG. 25A illustrates an exemplary flowchart for the present ribboninterconnect using conductive adhesive. Operation 280 places aconductive adhesive line along multiple adjacent bond pads, for example,by a printing process, with optional curing. Operation 281 positions aribbon interconnect on the conductive adhesive for contacting themultiple bond pads. After the ribbon interconnect bonds with themultiple bond pads, operation 282 separates the ribbon interconnect intoindividual interconnect wires, for example, by a laser cutting processalong the length of the ribbon interconnect between the adjacent bondpads.

FIG. 25B illustrates an exemplary flowchart for the present ribboninterconnect using laser welding. Operation 290 positions a ribboninterconnect on a plurality of bond pads. Operation 291 bonds the ribboninterconnect to the multiple adjacent bond pads, with or withoutskipping the spaces between the adjacent bond pads. The bonding processcan be an ultrasonic welding, or a laser welding process, travelingalong the adjacent bond pads by a moving laser assembly or a mirrorassembly. The laser welding process preferably runs along with adjacentbond pads without skipping, but an optional shutter can be used to skipthe welding of the spaces between adjacent bond pads. After the ribboninterconnect bonds with the multiple bond pads, operation 292 separatesthe ribbon interconnect into individual interconnect wires, for example,by a laser cutting process along the length of the ribbon interconnectbetween the adjacent bond pads.

In the foregoing variations in which both dice are face up, it will bedesirable to have the upper chip bonded to the lower one forreliability, so that undesirable thermomechanical stresses are notplaced on the I/O connections. A variety of nonconducting adhesivesknown in the industry as die-attach adhesives, as well as many otheradhesives, may be used in a very thin film to bond the two chips durablytogether. These adhesives would be advantageously applied by inkjetprinting or other droplet dispensing technique prior to the depositionof the thin upper die.

In an embodiment, the present invention discloses systems and methodsfor interconnecting bond pads between integrated components utilizingribbon interconnects, comprising positioning a ribbon interconnect overa plurality of bond pads, and separating the ribbon interconnect intomultiple interconnect wires, with at least one interconnect wiredisposed on a corresponding bond pad. In an aspect, separating theribbon interconnect comprises laser ablating the ribbon interconnect,and separating the ribbon interconnect into multiple individualinterconnect wires, with each interconnect wire disposed on acorresponding bond pad. An integrated component can have a beveled slopefor facilitating wire placement. Positioning the ribbon interconnect cancomprise printing the ribbon interconnect. In an aspect, the methodfurther comprises bonding the ribbon interconnect to the correspondingbond pads, such as one of laser welding, ultrasonic welding, andapplying a conductive adhesive. The conductive adhesive line can beapplied along the bond pads before positioning the ribbon interconnect.Alternatively, the ribbon interconnect can be welded after positioningthe ribbon interconnect on the bond pads. In an aspect, a same laser isused for welding and separating. In an aspect, the integrated componentis positioned in the vicinity of another integrated component in an atleast partially stacked configuration, or in an at least partiallycoplanar configuration. At the other end of the ribbon interconnect, aconductive adhesive line can be applied or the ribbon interconnect canbe welded to the bond pads of the another component.

In an embodiment, the present invention discloses a method forinterconnecting bond pads between integrated components, comprisingapplying a conductive adhesive line along multiple bond pads of anintegrated component before positioning and bonding a ribboninterconnect on the conductive adhesive line over the multiple bondpads. The ribbon interconnect is then cut by laser ablation intomultiple interconnect wires, with at least one interconnect wireconnecting on a corresponding bond pad. In an aspect, applying aconductive adhesive line comprises printing a conductive adhesive line.In addition, the conductive adhesive line can be cured. A pushingelement can push the ribbon interconnect onto the bond pads. In anaspect, separating the ribbon interconnect separates the ribboninterconnect into multiple individual interconnect wires, with eachinterconnect wire bonded on a corresponding bond pad.

In an embodiment, the present invention discloses a method forinterconnecting bond pads between integrated components, comprisingpositioning a ribbon interconnect over a plurality of bond pads, thenlaser welding the ribbon interconnect to the bond pads, and laserablating the ribbon interconnect into multiple interconnect wires, withat least one interconnect wire bonded on a corresponding bond pad. In anaspect, the laser welding runs continuously along the adjacent bondpads. Also, a same laser can be used for welding and ablating.

In an embodiment, the present invention discloses an integratedcomponent comprising a plurality of interconnect wires bonded to thecorresponding bond pads, with the interconnect wires cut from a ribboninterconnect into individual wires. In as aspect, the interconnect wiresare bonded to the corresponding bond pads by laser welding.Alternatively, the interconnect wires are bonded to the correspondingbond pads by a conductive adhesion line.

In an embodiment, the present invention discloses a chip assemblycomprising two integrated components connected by a ribbon interconnecthaving been separated by a laser ablation process. The two integratedcomponents are located in the vicinity of each other, such as positionedin an at least partially stacked configuration or in an at leastpartially coplanar configuration.

In an embodiment, the present invention discloses improvements for faceto face bonding between two chips. In an embodiment, the presentinvention discloses a surface treatment of the chip surface to confinethe bonding to the bond pads. In an aspect, the surface of at least onedie is treated in such a way that a liquid metal, such as Ga forexample, will not wet or stick to it, while the metallization of thecontact pads uses a metal which is wet by the liquid metal. When thesecond die is placed face down on this die, the thin film of liquidmetal makes a thin, uniform electrical contact between the two surfaces.If the liquid metal has been mixed with suitable fine particles (forexample Cu and Ni in Ga) which alloy to form a higher-melting pointmetal, a bond may be formed as the alloying reaction proceeds.

FIG. 26 illustrate an exemplary process of the present surfacetreatment. In FIG. 26A, a top chip 80 having bond pad 300 is coated witha liquid metal 302, and faces down toward a bottom chip 81 which has abond pad 301. The surface of the chip 81 is treated, for example, bycoating with a layer 303 that repels, non-wets or non-sticks to theliquid metal 302. The coating layer 303 covers the surface of the chip81, except the surface 304 of the bond pad 301. The surface 304 can alsobe coated with a liquid metal (not shown). The other surface of the chip80 (the surface that is not covered by the bond pads) can also betreated, for example with the same coating 303 (not shown).

In FIG. 26B, the chips 80 and 81 are brought together to form contactbetween the bond pads 300 and 301. The surface treatment 303 repels theliquid metal, confining the liquid metal to the bond pads 300 and 301,contacting the surface 306 between these two bond pads. In FIG. 26C, abond 307 is formed between the two bond pads 300 and 301 of chips 80 and81. Thus the surface treatment helps improve the pad bonding, forexample, by confining the bonding material to the bond pad surfaces.

FIG. 27 illustrates an exemplary flowchart of the present surfacetreatment. Operation 310 treats the surface of at least one chip (ordie), except the surface of the bond pads, so that a liquid metal willnot wet or stick to it. The surface of the other chip can be similarlytreated. Operation 311 coats the surface of a bond pad with a liquidmetal. The bond pad can belong to the chip with the surface treatment,or belong to a second chip, with or without the surface treatment.Alternatively, the surfaces of the bond pads of both chips can be coatedwith a liquid metal. The order of these two operations can be reverse,for example, coating the bond pads with a liquid metal before treatingother surfaces. Operation 312 contacts the surfaces of the two chipstogether to form bonds between the bond pads. Optional curing processcan be performed.

In an embodiment, the present invention discloses systems and methodsfor interconnecting bond pads between integrated components, comprisingtreating the surface of at least one integrated component, except thesurface of the bond pads, so that a liquid metal will not wet or stickto it, coating a bond pad surface with the liquid metal, and thencontacting the surfaces of the two integrated components so that thebond pads of the two integrated components are aligned, and wherein theliquid metal makes electrical contact between the two surfaces of thebond pads. In an aspect, the surfaces of both integrated components aretreated.

In an embodiment, the contact can be formed by dispensing molten solderdroplets, for example, in the same way to dispense conductive adhesive.The molten solder droplets can be dispensed by a heated printhead, or bya heated dispenser head.

In an embodiment, the present invention discloses capsules of liquidmetal covered by a polymer shell to improve contact bonding processbetween two chips. In an embodiment, vesicles or capsules of liquidmetal (e.g. Ga, with or without nanoparticles of other metals) areformed and surrounded by a polymer shell (from an emulsion process, orby self-assembly of polymer chains as surfactants). In an aspect, thepolymer has a specific affinity for the contact pads. Thus a monolayerof these capsules can form at the locations of metal contact, thencomplementary components can be placed against this vesicle surface.After squeezing the two components, the force breaks the vesicle andallows the metal to react.

FIG. 28 illustrate an exemplary process of the present capsule contactbonding. In FIG. 28A, a top chip 80 having bond pad 300 faces downtoward a bottom chip 81 which has a bond pad 301. The surface of thebond pad of chip 81 is coated with vesicles or capsules of liquid metal321, which are covered with a polymer shell 320. The polymer preferablyhas an attraction to the bond pad surface. In FIG. 28B, the chips 80 and81 are brought together to form contact between the bond pads 300 and301. In FIG. 28C, the capsules 321 are broken, forming a bond 327between the two bond pads 300 and 301 of chips 80 and 81.

FIG. 29 illustrates an exemplary flowchart of the present capsulecontact bonding. Operation 330 prepares capsules of liquid metal, whichare covered with a polymer shell having an affinity to the bond padsurface. Operation 331 coats the surface of a bond pad with the capsulesof liquid metal, wherein the polymer helps in confining the capsules tothe bond pad surface. The coating can be on one chip, or on both chips.Operation 332 places a complementary chip against the polymer shellsurface, with the alignment of bond pads from both chips. Operation 333squeezes the two chips together, breaking the polymer shells andallowing the liquid metal to react, forming bonds between the bond pads.An optional curing process can be performed.

This invention provides for the formation of RFID tags with stackedarchitectures, in which different circuit functions may be separatedaccording to fabrication cost and electrical performance to yield tagswith all of the performance appropriate to the application yet at alower cost. While the invention has been described with reference toRFID tags, it is not limited to any electrical function; it is equallyapplicable to any type of circuit in which more than one die is usefullystacked on at least one other die. Particularly useful applicationsinclude wireless sensors and other types of small scale integratedsystems, in which the dice are relatively small (e.g. less than a fewmm), or in which is desirable to have high densities of I/O connections.

In an embodiment, the present invention discloses systems and methodsfor interconnecting bond pads between integrated components, comprisingcoating a bond pad surface of an integrated component with capsules of aliquid metal, the capsules having a polymer shell, contacting thesurfaces of the two integrated components so that the bond pads of thetwo integrated components are aligned, and squeezing the two integratedcomponents to break the capsules, allowing the liquid metal to makeelectrical contact between the two surfaces of the bond pads. In anaspect, the capsules are formed from an emulsion process, or byself-assembly of polymer chain as surfactant. Also, the polymer can havean affinity with the surface of the bond pad.

1. A method for interconnecting bond pads between integrated components,the method comprising: forming interconnect wires on a thermallydecomposable adhesive on a printhead substrate; positioning theprinthead so that at least an interconnect wire of the interconnectwires is located over a desired location of bond pads; releasing the atleast an interconnect wire, the at least an interconnect wire falling onthe corresponding bond pads.
 2. A method as in claim 1 furthercomprising forming a beveled slope of one of the integrated components.3. A method as in claim 1 further comprising pushing the at least aninterconnect wire onto the bond pads.
 4. A method as in claim 1 furthercomprising laser welding the at least an interconnect wire to the bondpads.
 5. A method as in claim 1 further comprising applying a conductiveadhesive on the bond pads before releasing the at least an interconnectwire.
 6. A method as in claim 1 wherein the integrated components arepositioned in an at least partially stacked configuration.
 7. A methodas in claim 1 wherein the integrated components are positioned in an atleast partially coplanar configuration.
 8. A method as in claim 1wherein the printhead simultaneously releases multiple interconnectwires over corresponding bond pads.
 9. A method for forming interconnectwires on a printhead, the method comprising: placing interconnect wireson a thermally decomposable adhesive on a printhead substrate; lasercutting the interconnect wires to correct lengths.
 10. A method as inclaim 9 wherein the interconnect wires are separated by a same distanceas the separation of adjacent bond pads.
 11. A method as in claim 9wherein multiple interconnect wires are cut to a same length forsimultaneously printing on multiple bond pads.
 12. A method as in claim9 further comprising coating a layer of thermally decomposable adhesiveon the printhead substrate.
 13. A method as in claim 9 wherein thethermally decomposable adhesive is releasable by at least one of heatand light.
 14. A method for forming interconnect wires on a printhead,the method comprising: depositing a conductive layer on a thermallydecomposable adhesive on a printhead substrate; patterning theconductive layer to form interconnect wires with correct lengths.
 15. Amethod as in claim 14 wherein patterning the conductive layer comprisesa photolithography process.
 16. A method as in claim 14 wherein theinterconnect wires are separated by a same distance as the separation ofadjacent bond pads.
 17. A method as in claim 14 wherein multipleinterconnect wires are patterned to a same length for simultaneouslyprinting on multiple bond pads.
 18. A method as in claim 14 furthercomprising coating a layer of thermally decomposable adhesive on theprinthead substrate.
 19. A method as in claim 14 wherein the thermallydecomposable adhesive is releasable by at least one of heat and light.20. A method for forming interconnect wires on a printhead, the methodcomprising: extruding an interconnect wire on a thermally decomposableadhesive on a printhead substrate; laser cutting a correct length of theinterconnect wire, the interconnect wire falling on the thermaldecomposable adhesive.
 21. A method as in claim 20 wherein extruding theinterconnect wire comprises a mechanical feeder.
 22. A method as inclaim 20 wherein extruding the interconnect wire comprises a wirebonderwire feeder mechanism.
 23. A method as in claim 20 wherein theinterconnect wires are separated by a same distance as the separation ofadjacent bond pads.
 24. A method as in claim 20 wherein multipleinterconnect wires are cut to a same length for simultaneously printingon multiple bond pads.
 25. A method as in claim 20 further comprisingcoating a layer of thermally decomposable adhesive on the printheadsubstrate.
 26. A method as in claim 20 wherein the thermallydecomposable adhesive is releasable by at least one of heat and light.27. A printhead for printing interconnect wires, comprising a thermallydecomposable adhesive on a printhead substrate; a plurality ofinterconnect wires patterned to the correct lengths disposed on thethermally decomposable adhesive; and a release mechanism for releasingthe thermally decomposable adhesive.
 28. A printhead as in claim 27wherein the interconnect wires are separated by a same distance as theseparation of adjacent bond pads.
 29. A printhead as in claim 27 whereinmultiple interconnect wires are cut to a same length for simultaneouslyprinting on multiple bond pads.
 30. A printhead as in claim 27 whereinthe release mechanism comprises at least one of heat and light.
 31. Aprinthead as in claim 27 wherein the interconnect wires are placed onthe thermal decomposable adhesive and laser cut to the correct lengths.32. A printhead as in claim 27 wherein the interconnect wires arepatterned to the correct lengths from a deposited conductive layer onthe thermal decomposable adhesive.